Garden formal is fancy wear that is not inappropriate for outdoor terrain.
Women should wear dresses or a nice shirt/skirt combo but pay attention to length. You won’t want to wear anything too long because it could get muddy, and nothing too short since it is hard to pull off short and formal. The dress or skirt should be made of nice fabric and look like it was made to wear at a special occasion, which it likely is if you were told that the dress code is garden formal. Also, it’s usually a good idea to avoid high heels if you are going to be outside.
For men a nice suit and tie is preferred. Depending on the weather a suit jacket can be worn or left on a chair. Again, you should look nice for the occasion (NO JEANS OR SHORTS!!).
Another way to define it is casual semi-formal.